## Current, Voltage & Resistance

The three controlling factors always present in operating electric circuits are:

Current: A progressive movement of free electrons along a wire or other conductor, caused by the presence of electrostatic lines of force (water flowing through a pipe)

Electromotive force: The electron-moving force in a circuit that pushes and pulls electrons (current) through the circuit (water pressure)

Resistance: Any opposing effect that hinders free-electron progress through a conductor in a circuit when an electromotive force is attempting to produce a current in the circuit (similar to resistance to water flow caused by rough pipe walls)

Changes in the values of any one of these "big three" will produce a change in the alue of at least one of the others. Not their interrelationships in a simple electric circuit.

The amount of current in a circuit is basically measured in amperes, abbreviated A or amp. An ampere is a certain number of electrons passing a single point in an electric curcuit in one second. Therefore, an ampere is rate of flow, similar to gallons per minute in a pipe.

Voltage is responsible for pulling and pushing of the electric current through a circuit. The unit of measurement of electric pressure is the volt (V). A single flashlight dry cell produces about 1.5V. A volt can also be defined as the pressure required to force a current of one ampere through a resistance of one ohm (measurement of resistance).

The resistance a wire or other conducting material will offer to a current depends on these factors:

1. The type of material from which it is made

2. The length (the longer, the more resistance)

3. Cross-sectional area of the conductor (the more area, the more molecules with free electrons, and the less resistance)

4. Temperature (the warmer a conductor, the more resistance)

The unit of measurement of resistance is the ohm.

Scientific measurements often use the metric system which is a multiple of 10 system. The basic units are

length: meter

volume: liter

weight: gram

Volts, amperes, ohms, etc. may also use metric based prefixes. The prefixes in general use in electronic work:

micro = millionth = 10^-6 example uV (microvolt) a millionth of a volt

milli = thousath of = 10^-3 example mA (milliampere) 1/1000 A or 0.001 A = 1 mA

dB (decibel) 1/10 B (bel) or 0.1B = 1 dB

Hz = cycles per second = 60 Hz = 60 cycles per second

Law that applies to electric circuits; Ohm's Law

Wherever electric circuits are in use, voltage, current and resistance are present. It is interesting to see how the theory of more complex circuit operation unfolds by starting with a simple circuit and slowly adding one step to another. Once readers comprehend something of the physical nature of current, voltage, and resistance, they are ready to use this knowledge to learn when, where, how, and why these factors may be applied to electric circuits.

A change in current can be produced by changing either the voltage or the resistance in the circuit. An increase in voltage witll increase current. There fore, a voltage and current are directly proportional to each other.

An increase in resistance in a circuit decreases current. Therefore, resistance and current are inversely proportional to each other.

These two facts can be condensed into one statement known as Ohm's law: Current varies directly as the voltage and inversely as the resistance.

Ohm's law is a simple statement of the functioning of an electric circuit. It can be expressed mathematically as

I = V/R also written as I = E/R where I = intensity of current (in amperes, A); V = emf (in volts) (or E for emf); R = resistance (in ohms)

Ohm's law might also be expressed as Amperes = Volts/Ohms. I = E/R, E = IR, R = E/I

These three variations of the Ohm's law formula make it possible to determine 1 the current value if the voltage and resistance are known, 2 the voltage in the circuit if the current and resistance are known and 3 how much resistance is in the circuit if the votage and current are known.

An understanding of this law and an ability to use it are quite important.

Example: An electric shock of more than 15 mA flowing through the body is considered dangerous to human life. What current will flow through a person having a body-contact resistance of 2200 ohms across 110V? I = V/R = 110/2200 = 0.05A or 50mA. If the resistance in a circuit is increased 5 times and the voltage is tripled, what is the final current value? I = V/R = 3V/5R = 3/5 as much.

Alternating current

Power supplies

Filters

Digital Fundamentals

Amplifiers

Gain

Impedance -A value that is the resultant of the resistance and reactance will have to be determined. This will be called the impedance (Z) of the circuit, which is also measured in ohms. The effective resistance of an electric circuit or component to alternating current, arising from the combined effects of ohmic resistance and reactance.

Current: A progressive movement of free electrons along a wire or other conductor, caused by the presence of electrostatic lines of force (water flowing through a pipe)

Electromotive force: The electron-moving force in a circuit that pushes and pulls electrons (current) through the circuit (water pressure)

Resistance: Any opposing effect that hinders free-electron progress through a conductor in a circuit when an electromotive force is attempting to produce a current in the circuit (similar to resistance to water flow caused by rough pipe walls)

Changes in the values of any one of these "big three" will produce a change in the alue of at least one of the others. Not their interrelationships in a simple electric circuit.

The amount of current in a circuit is basically measured in amperes, abbreviated A or amp. An ampere is a certain number of electrons passing a single point in an electric curcuit in one second. Therefore, an ampere is rate of flow, similar to gallons per minute in a pipe.

Voltage is responsible for pulling and pushing of the electric current through a circuit. The unit of measurement of electric pressure is the volt (V). A single flashlight dry cell produces about 1.5V. A volt can also be defined as the pressure required to force a current of one ampere through a resistance of one ohm (measurement of resistance).

The resistance a wire or other conducting material will offer to a current depends on these factors:

1. The type of material from which it is made

2. The length (the longer, the more resistance)

3. Cross-sectional area of the conductor (the more area, the more molecules with free electrons, and the less resistance)

4. Temperature (the warmer a conductor, the more resistance)

The unit of measurement of resistance is the ohm.

Scientific measurements often use the metric system which is a multiple of 10 system. The basic units are

length: meter

volume: liter

weight: gram

Volts, amperes, ohms, etc. may also use metric based prefixes. The prefixes in general use in electronic work:

micro = millionth = 10^-6 example uV (microvolt) a millionth of a volt

milli = thousath of = 10^-3 example mA (milliampere) 1/1000 A or 0.001 A = 1 mA

dB (decibel) 1/10 B (bel) or 0.1B = 1 dB

Hz = cycles per second = 60 Hz = 60 cycles per second

Law that applies to electric circuits; Ohm's Law

Wherever electric circuits are in use, voltage, current and resistance are present. It is interesting to see how the theory of more complex circuit operation unfolds by starting with a simple circuit and slowly adding one step to another. Once readers comprehend something of the physical nature of current, voltage, and resistance, they are ready to use this knowledge to learn when, where, how, and why these factors may be applied to electric circuits.

A change in current can be produced by changing either the voltage or the resistance in the circuit. An increase in voltage witll increase current. There fore, a voltage and current are directly proportional to each other.

An increase in resistance in a circuit decreases current. Therefore, resistance and current are inversely proportional to each other.

These two facts can be condensed into one statement known as Ohm's law: Current varies directly as the voltage and inversely as the resistance.

Ohm's law is a simple statement of the functioning of an electric circuit. It can be expressed mathematically as

I = V/R also written as I = E/R where I = intensity of current (in amperes, A); V = emf (in volts) (or E for emf); R = resistance (in ohms)

Ohm's law might also be expressed as Amperes = Volts/Ohms. I = E/R, E = IR, R = E/I

These three variations of the Ohm's law formula make it possible to determine 1 the current value if the voltage and resistance are known, 2 the voltage in the circuit if the current and resistance are known and 3 how much resistance is in the circuit if the votage and current are known.

An understanding of this law and an ability to use it are quite important.

Example: An electric shock of more than 15 mA flowing through the body is considered dangerous to human life. What current will flow through a person having a body-contact resistance of 2200 ohms across 110V? I = V/R = 110/2200 = 0.05A or 50mA. If the resistance in a circuit is increased 5 times and the voltage is tripled, what is the final current value? I = V/R = 3V/5R = 3/5 as much.

Alternating current

Power supplies

Filters

Digital Fundamentals

Amplifiers

Gain

Impedance -A value that is the resultant of the resistance and reactance will have to be determined. This will be called the impedance (Z) of the circuit, which is also measured in ohms. The effective resistance of an electric circuit or component to alternating current, arising from the combined effects of ohmic resistance and reactance.